n. In anatomy, the innermost digit of the hand or foot, when there are five; the thumb or the great toe, especially the thumb, the great toe being usually distinguished as pollex pedis, or hallux.n. In zoology: The innermost digit of the fore limb only, when there are five; the digit that corresponds to the human thumb.n. The thumb of a bird; the short digit bearing the alula or bastard wing, regarded as homologous with either the human thumb or the forefinger.n. The movable joint of the forceps of the lobster, crawfish, or crab; a dactylopodite.